This invention relates to a distortion compensating amplifier and, more particularly, to a distortion compensating amplifier for monitoring input of undesired waves from an antenna and exercising control so as to halt updating of a distortion compensation table in accordance with the level of an undesired-wave input, thereby improving the distortion compensation characteristic at the time of an undesired-wave input.
In wireless communications in recent years, there is growing use of high-efficiency transmission using digital techniques. In instances where multilevel phase modulation is applied to wireless communications, a vital technique is one which can suppress non-linear distortion by linearizing the amplification characteristic of the power amplifier on the transmitting side and reduce the leakage of power between adjacent channels. Also essential is a technique which compensates for the occurrence of distortion that arises when an attempt is made to improve power efficiency by using an amplifier that exhibits poor linearity.
FIG. 10 is a block diagram illustrating an example of a transmitting apparatus in a radio according to the prior art. Here a transmit-signal generator 1 transmits a serial digital data sequence and a serial/parallel (S/P) converter 2 splits the digital data sequence alternately one bit at a time to convert the data to two sequences, namely an in-phase component signal (also referred to as an “I signal”) and a quadrature component signal (also referred to as a “Q signal”). A DA converter 3 converts the I and Q signals to respective analog baseband signals and inputs these to a quadrature modulator 4. The latter multiplies the input I and Q signals (the transmit baseband signals) by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90°, respectively, and adds the results of multiplication to thereby perform quadrature modulation and output the modulated signal. A frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier 6 power-amplifies the carrier output from the frequency converter 5. The amplified signal is released into space from an antenna 7.
In mobile communications based upon W-CDMA, etc., the transmission power of the transmitting apparatus is a high ten watts to several tens of watts, and the input/output characteristic [distortion function f(p)] of the transmission power amplifier 6 is non-linear, as indicated by the dotted line in FIG. 11A. Non-linear distortion arises as a result of this non-linear characteristic, and the frequency spectrum in the vicinity of a transmission frequency f0 comes to exhibit a characteristic having side lobes, as indicated by the solid line SA in FIG. 11B, instead of the ideal characteristic indicated by the dotted line SI in FIG. 11B, leakage into adjacent channels occurs and this causes interference between adjacent channels. More specifically, owing to non-linear distortion, there is an increase in power that causes transmitted waves to leak into the adjacent frequency channels, as shown in FIG. 11B. ACPR (Adjacent Channel Power Ratio), which indicates the magnitude of leakage power, is the ratio between the power of the channel of interest, which is the area of the spectrum between the one-dot chain lines A and A′ in FIG. 11B, and the adjacent leakage power, which is the area of the spectrum between the two-dot chain lines B and B′, that leaks into the adjacent channel. Such leakage power constitutes noise in other channels and degrades the quality of communication of these channels. Such leakage must be limited to the utmost degree.
The foregoing relates to a case in which the transmitted wave is a single wave. In a situation where a signal is transmitted by a plurality of waves, e.g., four waves, the frequency spectrum in the vicinity of the center frequency f1 of the transmit signal is such that side lobes (distortion components) SL develop, as illustrated in FIG. 12A, signal leakage into adjacent channels occurs and interference occurs between channels.
Ideally, therefore, it must be so arranged that the characteristic becomes as indicated by the dotted line SI in FIG. 11B in case of a single wave and as indicated in FIG. 12B, i.e., a characteristic without side lobes, in case of four waves.
Leakage power is small in the linear region [see FIG. 11A] of a power amplifier and large in the non-linear region. Accordingly, it is necessary to broaden the linear region in order to obtain a transmission power amplifier having a high output. However, this necessitates an amplifier having a performance higher than that actually needed and therefore is inconvenient in terms of cost and apparatus size. Accordingly, a transmission apparatus that has come to be adopted is equipped with a distortion compensating function that compensates for distortion ascribable to non-linearity of the power amplifier.
FIG. 13 is a block diagram illustrating a conventional distortion compensating amplifier having a digital distortion compensating function (see the specifications of JP2003-8360A, JP2001-203539A, by way of example). A transmit signal x(t) is input to a distortion compensator 11 in the form of, e.g., I, Q orthogonal signals (baseband). The distortion compensator 11 includes a distortion compensation coefficient memory 11a (a distortion coefficient compensation table) for storing distortion compensation coefficients h(pi) (i=0˜1023) conforming to power levels pi of the transmit signal x(t); a predistortion unit 11b for subjecting the transmit signal to distortion compensation processing (predistortion) using a distortion compensation coefficient h(pi) that is in conformity with the power level of the transmit signal; a distortion compensation coefficient updater 11c for comparing the transmit signal x(t) with a demodulated signal (feedback signal) y(t), which has been obtained by demodulation in an orthogonal detector described later, and for calculating and updating the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will diminish, e.g., approach zero; a power calculation unit 11d for calculating the power of the transmit signal; a delay circuit 11e set to a delay time Tτ, which extends from the moment the transmit signal x(t) is input to the distortion compensator 11 to the moment the feedback signal y(t) is input to the distortion compensation coefficient calculation unit 11c, for delaying the transmit signal x(t) by this delay time; an FFT (Fast-Fourier Transform) unit 11f for outputting a distortion component, which is outside the transmit-signal frequency band, included in the feedback signal y(t); and a monitoring control circuit 11g for setting and adjusting the delay time Tτ based upon the power of the transmit signal and the power of the distortion component outside the frequency band.
The power calculation unit 11d of the distortion compensator 11 calculates the power of the entering transmit signal x(t), reads a distortion compensation coefficient h(pi) conforming to power pi (i=0 to 1023) out of the distortion compensation coefficient table 11a and inputs the coefficient to the predistortion unit 11b. The latter executes distortion compensation processing (predistortion) by multiplying the transmit signal x(t) by the distortion compensation coefficient h(pi) conforming to the power level of the transmit signal and outputs the resulting signal.
The signal (actually a complex signal) that has been subjected to distortion compensation processing by the distortion compensator 11 is input to a digital modulator (QMOD) 12. The latter applies digital quadrature modulation to the in-phase and quadrature components (I and Q signals) of the signal that has undergone distortion compensation processing, and a DA converter 13 converts the digital quadrature-modulated signal to an analog signal and inputs the analog signal to a frequency converter 14. The latter mixes the quadrature-modulated signal and a local oscillation signal, thereby up-converting the modulated-signal frequency to radio frequency. The radio-frequency signal is input to a high-frequency amplifier 15a of a transmitter 15.
The high-frequency amplifier 15a subjects the input signal to high-frequency amplification. The transmit signal that has undergone high-frequency amplification is input to an antenna 17 from a feeder line 16 via a distributor 15b and isolator 15c, and the signal is released into space from the antenna 17. Part of the transmit signal that is output from the high-frequency amplifier 15a is branched by the distributor 15b, which comprises a directional coupler, and is attenuated by an attenuator 15d and then fed back to a frequency converter 18. The latter down-converts the radio-frequency signal to a baseband signal and inputs this signal to an AD converter 19. The latter converts the baseband signal to digital data and inputs the digital data to a digital quadrature demodulator (QDEM) 20. The latter subjects the input signal to quadrature demodulation processing, reproduces the baseband signals on the transmitting side and inputs the result as the feedback signal y(t) to an error calculation unit (not shown) within the distortion compensation coefficient updater 11c. The latter compares the transmit signal x(t), which has been delayed by the delay circuit 11e, with the demodulated signal (feedback signal) y(t) obtained by demodulation in the digital quadrature demodulator (QDEM) 20, calculates distortion compensation coefficients h(pi) based upon an adaptive control algorithm so as to null the difference between the compared signals, and updates old coefficients in the distortion compensation coefficient table 11a by the calculated distortion compensation coefficients. In parallel with the above operation, the monitoring control circuit 11g calculates the ACPR from the power of the transmit signal and leakage power (the distortion-component power) that is outside the frequency band and adjusts the delay time Tτ based upon the ACPR calculated.
By subsequently repeating the above operation, non-linear distortion of the high-frequency amplifier 15a in transmitter 15 is suppressed to reduce leakage of power between adjacent channels, whereby the frequency spectrum becomes as illustrated in FIG. 12B.
FIG. 14 is a diagram useful in describing processing for updating distortion compensation coefficients by adaptive LMS (Least Mean Square). Components in FIG. 14 identical with those shown in FIG. 13 are designated by like reference characters. The power calculation unit 11d has a power measurement unit 21 for calculating power pn (=|x|2) of the transmit signal x(t), reads a distortion compensation coefficient hn(p) that conforms to the power pn (n=0 to 1023) output of the distortion compensation coefficient table 11a and inputs the coefficient to the predistortion unit 11b. The latter performs distortion compensation processing (predistortion) by multiplying the transmit signal x(t) by the distortion compensation coefficient hn(p) that conforms to the power level of the transmit signal. The high-frequency amplifier 15a (FIG. 13) amplifies the distortion-compensated transmit signal x(t) and transmits it from the antenna. Part of the transmit signal amplified by the high-frequency amplifier is input to the distortion compensation coefficient updater 11c in the form of the feedback signal y(t).
The distortion compensation coefficient updater 11c includes a subtractor 31 that outputs the error e(t) between the transmit signal x(t), which has been delayed by the delay time Tτ, prior to the distortion compensation thereof and the feedback signal y(t); a multiplier 32 that performs multiplication between the error e(t) and a step-size parameter μ; a complex-conjugate signal output unit 33 for outputting a complex-conjugate signal y*(t); a multiplier 34 for multiplying the distortion compensation coefficient hn(p), which has been delayed by Tτ in a delay unit 37, by y*(t), thereby outputting u*(t); a multiplier 35 for multiplying μe(t) by u*(t); and an adder 36 for adding the distortion compensation coefficient hn(p) and μe(t)u*(t), thereby calculating a new distortion compensation coefficient hn+1(p) and inputting it to the distortion compensation coefficient table 11a. The latter updates the distortion compensation coefficient hn(p), which conforms to the transmit-signal power |x|2 delayed by Tτ in a delay element 22 in the power calculation unit 11d, with the distortion compensation coefficient hn+1(p).
If the update control set forth above is described in terms of mathematical expressions, the calculations indicated below are performed.hn+1(p)=hn(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=hn(p)x(t)f(p)u(t)=x(t)f(p)=hn(p)y*(t)p=|x(t)|2where x, y, f, h, u, e represent complex numbers and * signifies a complex conjugate. By executing the processing set forth above, the distortion compensation coefficient h(p) is updated so as to minimize the difference signal e(t) between the transmit signal x(t) and the feedback signal y(t), and the coefficient eventually converges to the optimum distortion compensation coefficient value so that compensation is made for the distortion in the transmission power amplifier.
The conventional distortion compensating amplifier described above gives no consideration whatsoever to a situation in which a signal from another service or a signal of another system enters from the antenna side and mixes in with the feedback signal y(t) (see the dashed arrow A in FIG. 13). A signal from another service or a signal of another system enters from the antenna side is an unwanted wave in distortion compensation control. A frequency spectrum of an unwanted wave W0 and transmit signals W1 to W4 of four waves is as shown in FIG. 15.
If an unwanted wave thus mixes in with the feedback signal, the proper error e(t) cannot be output and the distortion compensation characteristic deteriorates.
Further, the prior art monitors the distortion-component power (leakage power) outside the band indicated in FIG. 11B or FIG. 12B and adjusts the delay time Tτ in a delay circuit or the like so as to reduce the power of the distortion component. With the conventional method, however, an unwanted wave is regarded as distortion-component power, the distortion-component power cannot be detected correctly, an erroneous adjustment of delay time is made, the distortion compensation coefficient table shifts from the optimum values and the distortion compensation characteristic deteriorates.